1. Field of the Invention
The present invention relates to a steel having an ultralow yield strength. A damping structure is necessary to suppress and control the vibration of an architectural structure when subjected to earthquake, wind and other disturbance in order to ensure the safety and comfort of an architectural structure. The steel according to the present invention is very useful in many applications as a structural steel having a high ability to absorb vibration.
2. Description of the Related Art
To provide a low yield strength, attempts have been made to attain a chemical composition as close as possible to that of pure iron, because alloying elements in steel increase the yield strength of the steel. For example, a yield strength of 98 MPa was attained by a sheet product of pure iron (C: 10 ppm, N: 10 ppm, P: 10 ppm, S: 10 ppm) produced by a vacuum refining and casting, as described in "Zairyoo To Purosesu (Material and Process)", vol. 2, 1989, p 2021. To further lower the yield strength, the impurity amount must be minimized. A yield strength of 40 to 50 MPa was attained by a pure iron having a purity of 99.999% or more refined by an ultrahigh vacuum electron beam floating zone melting process, as reported in "Bulletin of the Japan Institute of Metals", vol. 24, No. 5, 1985, p 376-385.
Hexagonal boron nitride particles are introduced into a steel by a process disclosed in Japanese Patent Application No. 3-143602, in which a powder mixture is prepared by mixing a powder of hexagonal boron nitride with an iron powder and is then admixed to a molten steel.
The steel described in "Zairyoo To Purosesu", vol. 2, 1989, p 2021 cannot be used as a structural steel for architectural structures, because the yield strength is not sufficiently low and because the product is in the form of a sheet. Although the steel reported in "Bulletin of the Japan Institute of Metals", vol. 24, No. 5, 1985, pages 376-385 has a sufficiently low yield strength, the use of the ultrahigh vacuum electron beam floating zone melting process not only significantly raises the production cost but also is unsuitable for mass production. The steel disclosed in Japanese Patent Application No. 3-143602 is a free-cutting steel, which is different in application from an ultralow yield strength steel.